A few months ago I rescued a rat, his owner couldn't keep him any more due to a change in accommodation, I think the rat is about 6 months old now.

I bought two new rats with the intention of introducing them so that the original (Smoky) is not lonely when I'm asleep/at work.

The new guys (Fred and Ben, I wanted to call them Fred and Nobby but the missus wouldn't let me) are 7 weeks old and are now in a smaller cage next to Smoky's cage. The new guys seem very interested in Smoky, they are always on the side of the cage closest to his and they watch him constantly when he's near. Smoky is interested in them too, but he hisses at them and scrabbles his paws through the bars as if he is trying to reach them. Is this something to worry about in the future when I let them out together?

They have only been in the same room for one day now and I don't plan to introduce them outside their cages until next weekend, I'm planning to do it in the bath so they are in neutral turf and can't escape.

I really can't afford another cage if they can't get along, the new guys are in the travel cage at the moment but they will quickly outgrow that.