It's a semi-viable solution for the size issue, but for something like a Petal rogue he'd probably be better off neglecting serious weapons and using a pair of daggers, since all of the finesseable weapons have negligible damage dice at that size compared to the sources of bonus damage he can get. Though honestly it would just be easier to get (as suggested) a Dire Badger instead.
Also also, I would not advise TWF with two rapiers; you need to use a light weapon for your off hand to cut down on your attack penalties, and your damage as a TWF isn't going to be coming from an extra average 1 or 2 points of damage a weapon, it'll be from various sources of bonus damage (sneak attack for a rogue).
Ah yes, thank you - I think this is a left over idea of what is best to do after Immaterial's houserule, forgot about it.
Alternatively, you can use a double weapon, which is treated as a mainhand weapon + a light offhand weapon if you use it for TWF; a small two-bladed sword would give you 1d6/1d6 and 19-20/x2 crit.
Though do keep in mind a couple other things: if you're going to invest in TWF, you almost certainly want to get Pounce one way or another, so that you can do your full attack even when you have to move to get to your target. There are a couple ways to do this, but the two easiest are to either take the
Travel Devotion feat (CC p.62), which gives you 1/day full movement as a swift action, or by dipping one level of Barbarian in the spiritual totem variant from Complete Champion with the Lion Totem (CC p.47), which gives up Fast Movement in exchange for Pounce. If you don't know already, Pounce lets you full attack after a charge.
The rage is useless for you, but if you're thinking about also doing ranged stuff, you could see if our gracious DM will let you gestalt that with the
variant from UA which trades rage for ranger class features; a 1-level dip would give you a favored enemy, which will help your damage, even if you also CC into ranger, since it'll essentially be an extra FE.
Likewise, if you're going to go into TWF with a rogue, one of the viable paths to go is a Cunning Outlaw Rogue/Swashbuckler; a lot of the typical builds are something like Rogue 3/Something Else 1/Swash 16, Rogue 4/Swash 16, so that you're getting the sneak attack damage from the rogue and all the other much nicer class features from the swashbuckler. As an added bonus, Rogue 3/Spiritual Totem Lion+UA-variant Barb 1/Swash 16 would give you three different sources of bonus damage: sneak attack, favored enemy, and the Insightful Strike bonus damage from INT (in
addition to STR, rather than instead of) for finesseable weapons that you get at Swash 3.
It's actually rather unfortunate; Complete Scoundrel has feats to combine class levels for Rogue/Swashbuckler, Rogue/Scout, and Ranger/Scout, there's one in CA for Rogue/Monk, and IIRC even a dragon magazine with one for Rogue/Cleric, but no Rogue/Ranger that I'm aware of. Honestly, unless you're dead set on dealing with traps (and if you aren't, I've got that covered), a straight Ranger would probably be a better sneaky type for everything save sneak attack damage than a Rogue, which in and of itself is not overly attractive, given that one of the major factions here is composed entirely of undead. S'ultimately your decision, though.