The rule of thumb is not doing anything crazy before a major loan. One or 2 credit cards shouldn't be a problem, though.

If you don't like/can't travel much, I wouldn't bother with miles. Focus on cashback cards, and get only as many mile/point cards that you can realistically use within the next year. Apparently, since you do use Southwest, grab all 50K bonuses as they become available (they will), and work your way into a companion certificate (or 2).

Finally, if you are taking your parents to Hawaii, consider renting a 2-3 bdrm condo. There is an abundance of opportunities, and they are very reasonable. Seriously, Hyatt and Fairmont are good and dandy, but you won't get anywhere trying to accommodate so many people.

If you live on or close to the West Coast, your UR points transferred to BA can work miracles to Hawaii. One personal piece of advice: if you want to take your parents to Hawaii, do it sooner rather than later, because nobody gets younger.