You can, but it tanks reliability and competes with land doctrine for army xp (about two doctrines worth to get cost and reliability maxed). It's definitely a good idea to make your mech cheaper, but it does have its drawbacks. Unless mods are making army xp gain much more plentiful than vanilla, where it's pretty hard for most countries to have their land doctrine finished until probably 1942. And mech usually is getting researched in 39-40, so making a variant might set back your doctrine a decent bit. Honestly, there are a lot of things competing for army xp in vanilla compared to mp mods, where in particular template editing is usually made much cheaper or free. But in vanilla you're usually having to spend ~150-200 xp minimum on template editing, 200 making a mech variant, and more on army spirits.