All credit to you for wanting to have a go
and I can understand not wanting to invest too much just to test the water, but both your old flashguns and the budget fluorescent kit have serious limitations. You may get frustrated and give up, or if they do get you interested you'll soon want something better.
Flash guns suffer from a lack of a modelling light so you can't see what the light is doing (
very helpful, and will greatly enhance the learning process) and slow recycle time that is, perhaps surprisingly, pretty vital in portraiture. And you'll still need stands and softboxes/umbrellas, in addition to the triggering issues. Fluorescent lights need something around 500W for decent exposure levels or you'll be looking at marginal shutter speeds at low f/numbers and high ISO. And fitting different modifers is either difficult or impossible.
The far better option is studio flash, eg Lencarta Smartflash, with bright modelling light, fast recycle, and plenty of power when you need it. From around £100, and then you're into a proper flash system you can build on and take further.
That aside and given what you now have to hand, you could try optical slave triggering with the pop-up of your Nikon D610 on manual (disables pre-flash). Then get a box of Quality Street and put a couple of the dark red wrappers over the pop-up with a rubber band. That'll make it pretty much invisible but infrared will pass through and optical slaves have good IR sensitivity. My preferred option though would be cheap (manual, non-TTL) radio triggers (eg Yongnuo RF-603 transceivers - £20-ish a pair?). I doubt you'll have triggering voltage issues that seem to be 99% theoretical, the financial risk is very low and a decent set of radio triggers is always handy to have in any case (can be used with any flash gun or studio head, or as remote camera trigger with the right cable).
Let us know how you get on