1. Whenever a player is using a battle company and has included terminators or veterans into their force they are using members of the 1st company as well. This is normal and both fluflistically accurate and feasible since the 1st company rarely fights together as a whole. Mixing other battle companies however is a bit odd. If you were to have your primary company be a battle company and a few of your assault units be from one of the reserve assault companies it would make more sense. Alas, don't be discouraged from intermingling battlecompanies as they could be operating in the same theatre!
2. According to the SM codex page 28 a chapter maintains 20 something predators at any given time. Not sure on dreadnoughts. My BA second company has 4. I would imagine 1-3 being an average and acceptable amount but again this is my speculation with no flufflistic bearings.
3. Codex BA has a few pictures of their tanks. Both space marine tanks and dreadnoughts may have campaign badges in various locations. The placement and exact badge symbol if you choose one is up to you.
Land RaidersFor land raiders the front left and right side recesses have BA symbols as seen on my tank here. The decal location is courtesy of the previous incarnation of the LR box.
And another LR on the net.
DreadnoughtsFrom the dreadnought pictures and art I have seen I arranged my symbols as follows. Going from left to right you have the chapter symbol, company drop, and sarcophagous number. Some dreadnoughts I have seen (non BA) have their chapter symbol on the sides of the arms. I find this approach to be good when the right torso is heavily marred or covered in seals. Notice the campaign badge on the shin plate on both dreads.
And as though to shoot my previous statement down I offer another BA dread built and painted by JawaBalls. (Jawaballs, if you want me to remove the picture please PM me.)
And a Furioso dreadnought picture.
Rhinos and RazorbacksHere is a rhino. The flat side door without steps is best for the large BA symbol. In addition you could go with the traditional chapter symbol over the top hatch. The left fender (right fender in the picture) has the company drop while the right fender has the vehicle's squad number. In this case, this rhino belongs to tactical squad 4.
Predators, Vindicators, and WhirlwindsThese 3 heavy support tanks do not belong to a specific company as they are owned and loaned by the chapter's vehicle armory. These vehicles still sport the BA chapter symbol whether it be on the front fendors, side fendors, turrets, top of the hulls, dozer blades, side doors, or rear doors. As you can see in this picture the predator has a campaign badge on the left fendor. These are typically placed on the side of the tank towards the rear. For example you could place one above the back fendor on the side of the hull.
And here is a Predator from IA2.
I'm not sure about land speeder markings. I suggest flipping through codex SM if you have one available or looking online at GW's site or other people's work. I might be able to pull up a few pages from Imperial Armor to show you illustrations of these vehicles.
Okay, here is an IF landspeeder for reference.
I hope this offers some help.