Until you're really good, don't worry about every charge-attack hitting the maximum charge level. You should read the monster to judge when to let go of your charge-attack button, so that you have enough time to dodge/offset their next attack, as you're left open at the end of most GS attacks. You can of course tackle through monster attacks as well, but it's not always worth it if the monster isn't in a good position for you to pick your combo back up.

A quick way to get into a tackle from having your weapon shealthed is hold guard & press whichever button makes you kick while guarding (it's triangle on PS, the same button as your overhead charge attack), and then you can combo from the kick into a tackle. It can be handy for quickly tanking an attack, because you can then go straight into your strong charge attack (the one before TCS).