Well first of all, the Clans would be screwed.

Rusty would always be Rusty, and would never know Clan life. Cloudtail wouldn't join ThunderClan either, since Firestar wouldn't be there to take him from Princess.

In Firestar's Quest, Smudge has dreams about SkyClan. But if he was in ThunderClan, most likely another kittypet would dream about them instead.

Ok let's get into Into The Wild. Smudge would be the one to meet Greypaw, Bluestar and Lionheart instead of Rusty. And Rusty would probably try and stop him from going with them, since that's what Smudge did to Rusty in the original timeline.

Smudge would go with the cats into ThunderClan. He'd probably become Greypaw's best friend.

Now I like to think that Smudge is more soft than most kittypets. So I think it'll take him a while to adjust to Clan life. So I think that Sandpaw and Dustpaw would bully him more than they bullied Firepaw.

So I think they'll never be friends with Smudge. But Greypaw and Ravenpaw will.

I don't think Smudge would become a warrior in Into The Wild. I think it'd take him a while to complete his training, as he hasn't got the knack that Firestar had. But I think he would just about drive out Brokenstar and his rogues, with some help with the rest of the Clan, but it'd be harder for him than when Firepaw did it.

I also think he'd be kinder to Yellowfanf when he finds her. Since it was early into his training and he wouldn't have got used to the warrior code yet. He would've taken her to ThunderClan just as Firepaw had, but he wouldn't fight her first.

Also I don't think he'd have any suspicion about Tigerclaw until he tries to kill Bluestar. Smudge does seem a bit clueless to me.

Also for that reason, he wouldn't take Ravenpaw to the barn. Meaning Ravenpaw would be killed by Tigerclaw.

In Fire and Ice, I think that after he and Greystripe being WindClan back, he'd become a warrior. But I don't think he'd ready to take on an apprentice. Greystripe would still train Brackenpaw, but I think a cat like Mousefur would train Cinderpaw.

Speaking of Cinderpaw, she wouldn't injure her leg as that was Fireheart's idea for her to go to the Thunderpath. Instead, Bluestar would go, and probably injure her leg instead.

And because Smudge wouldn't have an apprentice to bring near the river, Greystripe would never meet Silverstream, who would never die.

Now onto Forest of Secrets. Tigerclaw would try to kill Bluestar, and Smudge would then see that Tigerclaw is evil. Since he didn't have an apprentice, he wouldn't be deputy. Whitestorm would be the next obvious choice.

Rising Storm. Smudge would probably save Yellowfang from the fire instead of Bramblekit, since he probably couldn't trust him after what Tigerclaw did. I don't know what would happen to Patchpelt and Halftail. They might still die.

In Dangerous Path, nothing changes. Swiftpaw and Brindleface still die, along with Runningwind, Whitethroat, and Bluestar too. Whitestorm would become leader, leading to the fact that we wouldn't die in Darkest Hour. He'd just lose a life.

I'm gonna guess that Whitestorm picked Brackenfur for his deputy, since he nearly was picked deputy once anyway.

SkyClan would remain lost, since no obvious cat could go and find them. Unless Whitestorm did it.

New Prophecy: Leafpool and Squirrelflight wouldn't exist. So their kits wouldn't exist. Or their kits' kits. And since Brambleclaw died in the fire due to Smudge not saving him, I have no idea who would go on the Great Journey to the sun-drown place. In terms of Hawkfrost trying to kill Firestar (who is a kittypet) he'd probably try and kill Whitestorm.

In Power of Three the Three don't exist so nothing really happens. Same with Omen of the Stars, other than the Dark Forest winning. Whitestorm will die and then Brackenfur will be leader!!

Vision of Shadows, SkyClan don't exist. And if they do, they won't join the other Clans. Or get separated.

So basically if Smudge did join, he'd be a background character, much like another Brackenfur.

Also fire would not save the Clans, since said fire would be at home snacking on some cat food in a bowl!