The Super Soaker 100 and 200 are very very easy to backpack mod. See - https://hydrowar.wordpress.com/2011/11/09/ss-210-mod/
Later I found that using the 200 reservoir as a PC for the 100 was way overkill. It's like using a Vindicator on the second smallest nozzle. You have to admit it looks pretty sick though.
I also made a backpack mod my nozzle modded (4x) 600. The 600 works really well with the new nozzle, but the backpack wasn't really the best idea. The 600 doesn't hold much in its reservoir, but its PC is quite large. With a stock nozzle this set up works well, but when you nozzle mod it the reservoir runs out fairly quickly. The backpack solves this issue, but makes it a little awkward like SEAL said. The 600 was VERY mobile before the backpack. So, I just took the backpack off, plugged the hole, and now use it stock. It just has to be refilled a decent amount.
I'm going to put a guide on the PPB. Please keep bugging me about that so I get around to it.
The PPB and Hydrocannon are truly the best candidates for backpacks. The 100 and 200 don't need it, but the way that they are designed (specifically the intake tubes) make these blasters a lot less awkward with backpacks than other guns (like the 600). The mobility on my 210 was still very high, much higher than my backpack modded 600, but the 100 needed a backpack less than the 600. Just some food for thought.
If you really want extra ammo in the form of a backpack I would look into just carrying around an Aquapak Devastator (unless you are referring to a PPB or Hydrocannon). The Devastator can be clipped to a belt or strap or whatever and doesn't get in the way. It can also be used as a sidearm to defend yourself while refilling or pumping.