Could I use an old 7' jig/worm rrod?

It depends on what the weight is your throwing, also w/j rods are know to float in weight class so that will depend also.

Figure the weight of the total rig (bait, hooks, style rig with how many arms) then follow the rod weight within the recommended spec

I've spec'd out an A-rig rod and chose a swimbait blank. It has more moderate action and heavier power than a normal jig/worm rod. If you'll be throwing a lighter rig once ina while you can probably get by with what you have. If you're throwing a 1 1/2> rig for a good part of the day a swimbait style rod will make it more enjoyable.

Could I use an old 7' jig/worm rrod?

Technically you could....I did....for 10 mins. I threw one on my 7'MH Zillion, rated the same as a 7'H Kistler and Powell. The rig was unweighted with 3-3/16oz heads and 2-3/8oz heads. I felt like I was going to snap my rod on a hard cast, and I've thrown many swimbaits on it. I plan on atleast using a flipping stick but probably my swimbait rod...something rated for atleast 2oz