In the first film we are shown how Alien can be injected into human body through human mouth. In Prometheus we can see another way an Alien can be injected into its host.
More specifically, we are shown an Alien can be injected during sexual intercourse.
Are there any limitations for the way they can get to human body?
I think the only real limitation to the insemination methods is the number of orifices on the host creature. In Alien vs. Predator: Requiem, we see the Predalien inseminating its hosts by injecting eggs/larvae/etc. down the host throats in a super wicked kiss of death. As long as there's a path to the stomach, at least, it looks like it can be done.
The Gibson script for the third movie includes other modes for reproduction. There are airborn spores (apparently invisible) which infect people who come into contact with them, and after some period of incubation their bodies rupture releasing multiple facehuggers.
This script was abandoned. I consider it superior to the film that was made. In it, Bishop, Hicks, and Newt survive. Ripley as well, though she is shipped off to Earth in statis, setting them up for a fourth movie where Hicks and Bishop return to an infested Earth to rescue her. But since this was unfilmed, it is hardly canon.
The facehugger in alien 3 is also different to normal facehuggers. its larger, clawed and capable of implanting multiple embryos into different hosts before it dies (minimum 2 as we see ripley and the dog become impregnated). Supposedly its a "Royal" facehugger, which implants a queen embro (the one in ripley) and one or more drones (the dog-alien) basically to protect the queen (also why the drone wouldnt attack ripley).
there are only 2 types of facehuggers the normal which plants 1 cancer and the royal facehugger which plants 2 cancers 1 praetorian which can evolve into a queen,carrier or ravager and also 1 drone,runner or soldier.
