Deadpool has been completely different creatures in different universes. A lot of other Marvel characters' backstories just vary from universe to universe. But Deadpool; he's completely different in some alternate universes.
In one he's a chick.
In another he's a dog.
So in the TF SG universe, Deadpool can be a Transformer!
Can't it stand to reason he can be a robot from Cybertron in the SG Universe, since he can be a chick in one and a dog in another? If he can cross species on Earth, why not cross species on other planets??
Transformers and Marvel have crossed over before. So it can stand to reason that at least one of the Marvel universes can intersect with one of the TF universes in some way.
(I just love Deadpool and want him around in some capacity for other TFs to play with, OK??)