^ The company seem to content with five-person group, so it's safe to say that they will continue with these five girls at least for a while.
Fortunately the Kenshuuseis are trained pretty well, so performance wise, there should be no problem if they decided to pick new girls from the talent pool.
About difference with MM, I believe that it's widely accepted that early years contracts are basically similar for everyone regardless the group. The amount of extra works is what makes the difference, like MM girls tend to have more live shows and events, more TV appearances, etc. But all things equal there's little or maybe even no difference in these newbies basic 'salary'.
Are these girls being paid enough? I remember Momoko saying she wanted a game console, as if she couldn't buy one.
I think UpFront is known to be one of the most 'generous' agency in the industry, so it should be at least 'reasonable'. Chisato for example, bought her father some gadgets each year like PS3 and SLR as his birthday presents just by saving up a little from her allowance for a few weeks. Or Reina went shopping and bought several LCD TVs for her home because she wanted it (and gave her brother 'x million yen' as new year gift).
They are seniors of course, with years of experience and pay rise through several contracts renewal, but I imagine for these young girls the amount of money offered by the company would be more than enough. For the parents though, things may be different..
Anyway, it's sad that Aina has to go. Being one of my favorite, it's really hard to take.