My first cat Grisette, a rescued feral, was maybe 3 months old when we got her and then she spent 3 weeks under the kitchen cupboard before she came out and became a nice and friendly cat.

Filou and Tigris ignored each other completely for one week although they were much younger than Ti and Miley are. Up to my opinion a little hissing an grrrring is not to bad because they notice each other and start to interact. It is a step forward compared to just walking away and finding different ways through the house.

I did never think Ti was fat: I just imagine that he must be quite higher, longer legs, longer body than little Miley. Tigris is very similar to Filou in shape -just every detail is bigger -from nosepad to tailtip. So he can weigh the double of Filou without being fat.