This is the bit that concerns me, not the idea of a 3 inch severum going into a 3 foot tank for a couple of months. It sounds like your going for atleast a month or longer.I'm leaving for Europe and for a while and my mom will be watching all my fish for me until June...
Is your mum going to doing water changes while your away? and not just feeding your fish.
Also what else is in your 30 gallon tank?