That's usually not the case. There's also a portal-speed that's different on each map, but creatures generally come slow enough that most rooms are researched by the time you reach your portal limit. Especially since Dragons and Warlocks (your best researchers) come straight away.
Build your library, train your researchers to lvl 2-4 (they have double the research rate at lvl 4 compared to lvl 1 and by that time they have their best spells) while you reinforce the walls of your library (an efficient room works faster) and start researching. Continue their training only after all rooms are researched.
Trolls have little value, Orcs are all right, but Mistresses are very good and they will be attracted to the torture room which takes a while to research. Together with Dragons and Bile Demons which are attracted by the starting room) they are the creature to get. There are some powerful heroes, and Vampires are also very good, but they don't come from the portal. Hell Hounds are usually the only creature you won't get normally because it is attracted by a room that is researched at the very end.
And remember, you can always throw creatures you don't want back into the portal to make space for new creatures. For example have the worthless creatures build doors (for increased room efficiency) or research new rooms (if you don't have warlocks shooting at them) until you reach the limit, and throw them into the temple or portal when you do so you get creatures you do want. This way you have an army if you are attacked in the mean time, but don't waste gold on training the weaklings.
I didn't know that I can throw monster in the portal.