The Warframe can be any Rank, but you cannot subsume a Prime Warframe. It is, however, perfectly fine to add a new ability to a Prime.

Can you helminth a prime Warframe?

As mentioned in the Update-Patchnotes of Update 29, Helmith does not accept prime Warframes.

Can you subsume a Warframe more than once?

Subsuming Warframes

. There is also a limited number of subsumed abilities the Helminth can hold, which can be increased with the Helminth's Metamorphosis rank. The same Warframe cannot be subsumed more than once.