I think they should add the more important and fitting characters first, so i say 18..
And why does everybody assume that 18 wouldn't be that strong?? She kicked Vegeta's ass when he was Super Vegeta! :warning:
If she can whipe the floor with a SSJ, but she's not as strong as Cell, i'd say she should be placed between Freeza and Cell.. Say 850.000 starting PL.. Remember that the Z fighters were no match for the androids (17,18) at first..
And the unlimited energy thing could be solved, it would just be more work to code i guess:
Give the androids a less fast decreasing KI bar, as in their moves would cost less energy.
Their charge animation wouldn't be really visible and would just be them standing there..
If you really have a problem with that you could make it so that instead of the charging like all the chars now do, they would have to stop attacking for a while (couple o seconds) and their KI bar would recharge (for example twice as fast as other chars) out of itself..
If you'd ~attack~ (Not move, teleport, fly) while the bar is recharging it would stop recharging and you'd have to wait again.. This would mean you'd have to think more about how your spend your energy..
Call it a battery instead of a KI bar for all i care.. :\
Like has been said before, their attack options could just expand when they reach a certain PL, without a flashy transformation..
As compensation for the smaller energy costs they would reach their second stage later than all the other char. So that would mean that choosing an android would give you the advantage in the beginning of the match, but you'd get a hard time as soon as your 'organic' opponent transforms.. (I know that this isn't exactly as in the show, but who says Krillin owning Majin Buu is? :S)
This is just an idea, but there are all kinds of chars in the show, all with their own advantages, so i really think that the game style with different chars should also be different.. And with that i mean # of transformations, advantages of those transformations, etc..