I am wondering if this feat would benefit my Summoner and his ability to summon monsters (both as a spell and spell-like ability) and let him cast the spell as if he were level 5 instead of casting at level 3 and does this add more monsters?
monsters don't come from caster level but from the spell itself, summon monster II is a completly different spell than summon monster IV.
Also I don't think spell specialization works on spell-like abilities.
The only effect spell specialization would have on summon monster, would be an increased duration and increased range. Also, it would have no effect on spell like abilities.