Hi Naj-Treb and welcome to Dakka, the "Any veteran my replace his chainsword and/or bolt pistol with:" just means that you replace 1 or both of the weapons that he comes with for a weapon listed in the upgrades, so if he comes with a Bolt Pistol and Chainsword you can keep the Bolt Pistol and get rid of the Chainsword for a power weapon or
LC, or you could get rid of both and take two
LCs, the only thing your doing is trading a weapon for an upgrade, as long as when you finish he doesn't have his Bolt Pistol and Chainsword along with 2
LCs then you're fine.
As to bikes over Razorback, as I run mainly Razorback/Rhino and haven't used bikes I can't really comment, but getting +1 toughness and relentless
USR is quiet attractive