Rick is an asshole, he's not better than the guy who tried to kill him. Let's face it, if a stronger group would march into his group's home and allow them to stay in exchange for half of their food, Rick would probably do the same thing, try to take out their leader.
If the series would follow another group of characters that would run into Rick's group they could easily be seen as the villains led by a crazy ex cop who executes everyone threatening his authority. I like that, it's much more realistic than a bunch of weaklings who shouldn't have a chance in this world.
Keeping the remaining prisoners alive is a mistake and I bet at least one of them will become a serious problem before this half season is over.
If someone had brutally murdered Rick and Daryl, hunting the latter down and having him eaten by zombies, would the rest of the group just sit in their cell block thinking "I guess they won"? No, they would observe, make plans and then strike when they think the time is right, if only to protect their half of the food that the five times larger group will come to get when they munched through their half.
No, Rick is NOT the same as Tomas. Lets remember that Rick and Co. freed the prisoners from the cafeteria and cleared out the infected prison. The prisoners would not have been able to do that themselves and in fact had no idea how important it would have been to do so. Rick was right that they spilled blood for that prison...which the prisoners were not able to use until they showed up and brought them up to speed on the new world order. That was not a situation of the Ricktatorship barging in and deciding to take someone else's home. They found a place, that no one else was using.
On top of that, Rick was perfectly willing to let them stay and even risked his life and that of his people to try and help THEM create a safe place in the prison. Instead it became abundantly clear that Tomas was going to try and kill Rick the first chance he got.
Also remember that the prisoners were not together by choice and did not necessarily follow Tomas' lead. They were just thrown together in that cafeteria. As Axel said, he had no reason to listen to Tomas, he was not their leader. Plus remember, the two remaining surviving prisoners had NO reason to show any allegiance to Tomas after he brutally killed Big Tiny right in front of them (someone they probably had more affection for anyway). Its a good bet that given a choice, Rick would have locked up Big Tiny or set him free and see what happens.