That should work fine. I have had very good success adding smaller fish into my larger tanks. The majority of the time the fish ignore a smaller newbie. I assume it is because they do not feel threatened. Example: 2 days ago I added a 7" Bleekeri into my 300 gal, the Hoga, Hartwegi, Hetrospila and the Bifa ignored it completely. BUT, the 6" female Hartwegi and my 7" female Atro, beat the piss out of it. Off to a different tank.
As for growth rate, they grow fairly quickly if given adequate water turnover and food. My male went from 7" to 12" in about 4 months.
