I really like the way Flareon looks, but it's a physical attacker, leaving it little use for in-game. Maybe keep your Flareon in storage until after e4 where you can get it to learn more appropriate moves. Since you have SoulSilver, Ninetales is your other option (unless if you want to restart with Cyndaquil.) Ninetales is a satisfactory fire type. (:
For your 6th Pokemon you could get Dratini if you are up to doing some difficult training. You already have a flying type, but Gliscor is a nice Pokemon and remains an option.
If you can trade, you can get a Donphan. It's slow but has good Attack.
Mamoswine might be useful against those pestful Dragons you have to face when you go up against Clair and the champion.
If you do think Execcutor is falling behind and you want a physic replacement, Espeon is good. Of course, you'll either have to breed you Flareon or trade to get one.