Boots, regardless of champion, are ALWAYS situational. The options for Sivir are Zerks, Mercs, Swift and Mobility. Any of them will work fine.
The reason we put Zerker boots in the list is, its the optimal option. If yuo play this strat well and its working for you, the Zerker boots will help you finish that inhib early where as the rest wont.
That being said, if your not dominating, you may want to look at the other choices. Of course, generally boots arent really discussed as they are pretty self explanatory. Too much CC on oposing team, get mercs. Enemy team pushing as fast as you are, get mobility. Pushing by yourself a lot, get swiftness (so you have a better chance of running).
So if your having better luck with mobility, thats fine. I personally dont take them because I find teleport does the job and I am pretty much ALWAYS incombat. Sivir is not a ganking champ that is constantly running between lanes.
I wouldnt even dream of not taking rally. It is, by far, the most powerful summoner spell on Sivir. It has many uses. Early game heal, teamfight boost and all that good stuff.
But there is one particular scenario where rally REALLY shines and I am SURE every single Sivir has run into it:
You got your starks, you 5 man pushed your lane and your staring at the enemies inhibitor tower and all 5 enemy champs. What to do. Well, generally you wait on the next creep wave and move in to attack the tower.
We have a problem of course. There are 5 enemy heroes killing creeps and attacking you. None of you want to attack the enemy champions and take tower fire. Well Rally actually solves all of these problems.
The first thing it does is gives your team courage. Everyone knows what a rally does. Youd be suprised how many champs (and players) KNOW they can down a tower very quickly with just a little extra damage and some defense while they do it. Rally provides this. Maybe that Yi who just wont run in will see the rally and know he cant get that tower down now. Maybe your alistar realises that between his abilities, ult and rallys heal that he can tank the enemy tower and all 5 heroes and finally engages. Whatever the case is rally will often give your team that extra push they need to go after the inhib tower.
The second thing it does is increase your teams damage by a LOT. Your Sivir! You have starks, you probably have your ult going! Adding 20 damage to each champion on TOP of the attack speed your giving them (and lifesteal) makes a HUGE difference. Drop your rally, run in and start nailing that tower! Your teamates WILL follow suit. The best thing about it is, your oponents will quicjkly see that the tower WILL go down and more often then not, one or two will run off to do something else.
Lastly that heal is incredible. Again, taking the inhib tower and the inhib it is crucial. I find that the difference in my games where we take that first inhib and then eed to go push another lane as oposed to the ones where we take the first inhib and roll right on through the nexus is Rally! 15 seconds while your knocking down the tower and the following inhib combined with starks 30/5 HP regen and the lifesteal you give to your team will keep them in fighting form! This keeps your push going and, more often then not, convinces the enemy carry that they shouldnt try to instagib one of your teamates and then run away (that is, in general, what stops a final push more then anything else).
In any case, that battle for the inhibitor tower is a defining moment of Sivirs game and NOTHING will help as much as a rally.
Aside from that its great in any teamfight. Yes the fight will move, but chances are thats because you dropped your rally, your team killed a couple of oponents and the enemy is now running away. Hell sometimes one of the oponents SEES the rally and decides that it would be best to back off and fight the teamfight away from the flag. Of course, he doesnt tell his teamates and it loses the teamfight for them. There are a LOT of scenrios like that where rally can be a deciding factor.
In addition it gives INCREDIBLE pushing power. I pretty much ALWAYS push the first mid tower at level 6 with a rally flag. You just march up with your creep wave, ignore the enemy champion, drop the flag, pop your ult and go to town on the tower. It WILL go down. I do of course try and send the eemy champ back (or kill them) first, but as long as they are low enough on health to be warry of you, you can easily take that tower with your ult and rally.
Lastly, it gives me about half of my early kills. A good half of those kills is on tristanas. All it takes is being around 1/3 health and tristana WILL jump you to engage. Drop rally activate potion, throw BB and then auto attack them to death. Works every time and you end the fight with full health and can then get some good damage in on the tower. It can also be used as a normal heal early on if you screw up or suffer a gank attempt and, believe it or not, it heals more then the summoner spell heal.
As to when you have scrubs for teamates. Chances are you probably still want rally. Why? Because while your scrub teamates are desperately trying to defend your base from the enemy teams push, you with this build and your rally can push just as fast, if not faster then all 5 enemy champions (faster if your teamates are dong anything to slow them down). Rally here again is huge and it has to do with the inhib tower and the inhib. Sivir with teleport and rally can backdoor the tower and inhib before the enemy can get there (short fo their own teleport, you will still kill the tower though, just not the inhib).
Im not a fan of fortify in general. I find the cooldown is WAY too long to be of any real use. Its doubly useless on Sivir though as you really shouldnt be fighting at your towers. It is too easy for Sivir to waltz into a lane and spend 3 seconds annihilating the wave to get the eb and flow of the waves moving towards the enemies base instead of yours. Early game its of little use to you because honestly, if your getting pushed to your tower as Sivir, your a teribad and Sivir is not for you. Its far too easy with her to push so you should never have your own tower in danger.
I feel like fortify is planning for failure. Id rather take something that can not only help defensively (Sivir guarding a tower with a rally ready is NOT losing that tower) as well as offensively (as we covered earlier).
I think the most difficult part of this strategy and accompanying build is when your playing good oponents who know that the inhibitor tower is the best place to stop your push. Rally is the only summoner spell that is really going to win that for you or at least tip the favor into your corner enough that either your teamates rally to the cause or the enemy flees at the sight of your war banner.
In the end though, your the danm Battle Mistress! Why wouldnt you carry around your very own inspiring battle standard!?!