The magnitude (power) of the enchant is determined by:
- the potency of the soul you're going to use (petty, lesser, common, greater or grand)
- your enchanting skill and the perks you've invested in
- the fortify enchanting potion you've potentially consumed before beginning the process and finally
- the buffs you're getting from specific quest rewards that fall under the spoilers category.
It's in no way determined by the item you disenchanted to learn the effect form.
As for the 2nd part of your question... do you want to stack 2x fortify carry weight effects on these gloves, or 1 carry weight & 1 fire resistance? The former is not allowed - the latter you can do.
And no -you can't disenchant what you've enchanted yourself, then use it to create quad enchants, if that's what you was after