Okay, that helps. Should have seen that myself. Summon nature's ally would get small elementals as a second level spell, as opposed to SM getting them at 3. Medium elementals don't come until SNA 4. Since you are limited to just one type of summons, I think it would be better to give them at the SNA levels instead of the SM levels. Maybe even kick mediums down to a level three spell. The strength of summoning spells is their versatility, which is severely hampered by limiting them to just one elemental type, so it doesn't seem crazy. Or you could just summon 1d3 small elementals instead.