Moto is one of the strongest decks you can get. Diggers are not cost effective as for 5 points more you get Commandos '90. Those are amazing.
All the things you need to understand the infantry are in the guides (look sidebar), and in the hidden knowledge spreadsheet. The spreadsheet is slightly outdated, but most of the inf was not changed.
VDV '90 and Spetsnaz will mop the floor with most of infantry cause spetsnaz has an anti infantry weapon instead of an anti tank one. So your infantry once in a while gets blasted in the face with 4he napalm round. Doesnt sound fun. The VDV '90 are there so Spetsnaz doesnt loose morale, and to destroy the enemy vehicle with their amazing at weapon. Something spetsnaz cant do.
With the french you should take Mistral Milan F3 Legion '90 RIMa '85 x2
But you shouldnt use the french deck. If you want france then take Euro corps.
There are two types of infantry based flamethrowers: The classic one and the rocket one.
So the rocket one is the thing assault engineers have, the thing spetsnaz and li jian '90 have and a few specialist squads like the assault engineers.
It fires, makes boom and burns enemy infantry.
The classical one is slightly different. It fires streams of fire with a goal of damaging enemy mostly. Then using guns to kill the weakened enemy. Regular classical flamers can win vs enemy elite infantry. As their rocket based counterpart they need to stop to fire. Usually you need to order them to attack (rmb on the enemy unit) or disable rifles for a second. Not sure how it happens but had that problem a few times.
You should use fire pos with flame tanks. They are usefull to burn buildings where you think enemy infantry is. It doesnt have to be spotted.
The important thing you need to know is there is 4 types of infantry:
Militia: Cheap, mostly useless unless used as fodder, dont bother with those without experience.
Regulars: Cheap, mostly useless with a few exceptions like Jager(and copies) or Bochongsu.
Shock: Core of your infantry. Not too expensive, eats line infantry and with a numbers advantage can win against enemy elites. Those units usually have good antitank weapons.
Elite: The best infantry you can get. Comes at a big price, but played correctly will always pay for themself.
While SAS/FJB-40 are elites they should never fight infantry. Good for flanking/ backdoors or early airborne push. LSTR is a better version of these two and can be used in second line for vampyr/igla support. Their rifle is capable of stopping shocks in 1:1 proportions, but they shouldn't be used as front line infantry.
There are also specialist squads (all regular except for polish saperzy szturmowi and spg-9d that are shock), but im not going to get into them.
My post is surely long and chaotic, so if you have any questions then feel free to ask.