Interaction with other cats

I have had Tomnus around 3 different cats. The first one is my parents cat at my parents house. Tom was very submissive and upon meeting this cat Tom was on his side showing that he wasn't a threat. My parents cat didn't really care about Tom so they get along okay. I've had him stay at my parents house with out me for up to 10 days and he has been fine with my parents cat. Then I tried to bring a second cat into the apartment and that didn't go well at all. I spent the entire day and night breaking up cat fights before I finally locked the other cat in the bathroom and then Tom still tried to get at him though the door. I returned that cat and Tom went back to normal after a few days. Recently I had him at my summer home for the first time with another cat. It was my grandmothers cat and again tom was very submissive. He went right up to the other cat and rolled onto his side again. The other cat didn't buy it this time though and growled and hissed at Tom but thats as far as it went.

I am wondering why he gets along with some cats and not others?

The summer home, though we aren't up there on a regular basis would still be considered his property because he has been up there about 6 times with no other cats so I am sure he see's it as his so I don't think its a territory thing. My parents cat and my Grandma's cat are both rather large(over 17 pounds each) and tom is about 9 pounds so it might be a size thing but I don't know. The cat I brought home was only about 5 months old where Tom was about 10 months old but he's big for his age(long and tall). The third guess I have is he somehow tells that the other two cats that he got along with were older and showed them submission because they were older but again I doubt that.

Any Ideas?

I may be wrong but it does seem like a territorial thing, it sounds like the only place you have problems is in your apartment, where he spends most of his time. While the other place is his too, he's not there as much as he is in the apartment. The other places are neutural territory. My sister has a dog that doesn't like other dogs but when he's in my back yard he's very passive if there's another dog there.

It's not about 'which' other cat, it's about where... the time Tom got angry was when another cat was in Tom's territory, so he defended it (and had the brains to be submissive when in other cats' places). If you want to bring in another cat to your place (permanently), then you need to do proper intros, not just bring it in and dump it. There's a kind of protocol and it can take about 2 wks (tho' occasionally less), where you keep the new one behind a door, but hopefully one that at least has a little air or space underneath for them to sniff each other, and/or one with a window. At certain times of the day you put Tom behind another door, and let the new one check out your place and leave his scent around for Tom to pick up later on, and so Tom understands that his territory is now shared. After a week or so, you let the new one out (when Tom's around) but stay close to referee if they actually fight (vs hissing and yowling). If they do fight, you need to make sudden loud noises (marbles or pennies shaken in a can work) to distract them long enough to get between them (another person would help, as would longish gloves) and get them back to their own rooms, or at least the new one. Then try things again the next day, or later on, etc. etc. but always keep up the separate visits to neutral territory (and always have at least two litter boxes of course, tho' the new one's box would be in his temp room of course). After a few days things should lighten up (I'm presuming both cats are fixed, BTW, as it'll be a waste of time otherwise). I would also suggest another male, tho' younger than Tom, and one of a somewhat similar temperment, rather than one either likely to try to dominate or else one who'd end up living under furniture forever. A female could work, but not as well, and it's quite possible Tom would always not only dominate but attack 'forever', never giving her any peace.