Leveling up skills you don't use has no bearing on the skills you do use. They will level up at the same rate no matter what. However, you need to keep in mind that Skyrim uses leveling scaling. So if you pump up a bunch of skills you don't use, like Smithing, and don't level up your main skills to match, you will no doubt run into some serious problems dealing with the higher level enemies.
You also need to consider that the only benefit to leveling up is you get an increase to either magicka, health, or stamina; and gain one additional perk point.
Perk points are useless unless you spend them, and perks have a skill level restriction, so if you only have 30 points into conjuration it doesn't matter how many perks you have, because you won't be able to use most of the perk tree anyways.
So in summary, although it is possible to power level by using skills like smithing, unless you plan to use those skills or are in desperate needs of either some perks or health/stamina/magicka, I would suggest against it. This doesn't mean you shouldn't put points into those skills you don't plan to put perks into, but you should be careful about gaining too many levels by power leveling those perks.
Best Answer
Casting spells in combat gives skill xp equal to the base cost of the spell.
If you use a Weapon spell, you can't recast while the weapon is out. Sheath it to cancel the spell, allowing the next cast.
If you use a Summon spell, it might kill the enemy, ending your combat, requiring you to search out another enemy to gain skill on.
If you use a Target spell, it must connect with a target to award skill xp.
So - to gain skill xp the fastest - enter combat and cast the most expensive conjuration spell you can. There are many good choices for spells, the problem becomes more about having enough magicka to cast it again. Use conjuration enchanted gear to modify conjuration costs (if you can get to 100% free cost, that's ideal). Use the Elf racial to regen magicka. Use potions. Take the 50% discount perks.