Shadow conjuration can mimic any sorcerer or wizard conjuration (summoning) or conjuration (creation) spell of 3rd level or lower.
(Shadow conjuration spell description)
A shadowcraft mage can use the altered spell to mimic any sorcerer or wizard conjuration (summoning), conjuration (creation), or evocation spell at least one level lower than the illusion spell. The altered spell functions identically to the shadow conjuration or shadow evocation spell, except that the spell's strength equals 10% per level of the figment spell used.
(Shadowcraft mage—shadow illusion (Su), Races of Stone)
You need a specific conjuration (summoning) spell to get the creature you want—it doesn’t have to be summon monster, so you can access things on other lists, but it has to be some actual conjuration (summoning) spell (from the sor/wiz list, within the level limit). You cannot create any creature ever and have it work.
I don’t believe there is any other way to extend shadow conjuring to allow other creatures. The shadowcraft mage prestige class is the strongest amplifier of those spells—by far—and it doesn’t do it. There aren’t any other illusion (shadow) spells you can use instead, either; shadow conjuration and greater shadow conjuration are the only spells that work like that.
You can, of course, create a regular illusion of any creature you like. Major image and anything higher-level than that can manage a very convincing illusion—but it can’t actually hurt anything.
