You only need a 55 or 75 you could actually use the 100 and set the baffles in there so you can use it as a grow out tank as well as a sump. Look up some refugium setups
It certainly isn't too big, you can never have too big a sump!!
"Is it necessary?" is the real question, you could quite likely get away with something smaller but having more space to play with would be cool. For me it would depend on the dimensions, if it is the same footprint as a 75g just taller I wouldn't use it, but if it gives you more length to play with them it could be handy.