The inf squad should take a grenade launcher rather than melta as their BS3, the GL range would be closer to the AC too. you will need a platoon command squad in order to have infantry squads, I would give them 1 vox, 2 flamers, 1 HF that could count as a flamer.
Depending on the role of your company command, you could give them 3 plasmas, 1 medic; lascanon, vox, flag; etc.
On the russes:
1 executioner(plasma cannon one) with plasma sponsons, the hull and sponson weapons can be changed easily in the kit
1 new battle cannon, make sure its the new kit, as it will allow you to mount a vanquisher cannon too, sponsons and hull interchanable
the other 2 you can go punisher/pask with all HBs, exterminator with all HB, demolisher/battle cannon with no sponsons, eradicator with HBs, conquerers(use battle cannon with stormbolter count as, model OOP from FW) with HBs, conquerer is a fast tank, it can move 12" and fire, demolisher can be used as thunderers count as without any hull and sponson weapons
the hydra can be easily swap between itself and a wyvern, just cut the ears that holds the hydra cannons in place, and swap the ammo around; use the griffon as a wyvern so you have a battery of 2.
If you buy FW stuff, get vulture with punisher cannon, sabres with TL lascannon, vendetta conversion kit if you magnetize your Valkyrie.