i kept a crocea and a max in a ~3 gallon nano with 4x9w PCs for over a year with no problems. they grew and had great color. even better than the ones in my 2x250DE 50g.
even growing at full speed, it'll take plenty of time for any clam to outgrow a 10g (other than maybe a gigas). get the clam, enjoy it for a year or two or three, then move it to one of your bigger tanks. no big deal.