...The pacman is going to have at least 7" above the tank to accommodate for a 5.5" ALT's heat lamp on top of it. I saw a Zoomed dome light but the dimensions look to be 10" high. I don't want the total height of this to be greater that 72".
With a 5.5" clearance you can look at either a mini dome (requires a dedicated small quartz bulb) or one of Zoo Med's Naturalistic Hood's.
The tomato frogs I am getting a Amazon.com: Zoo Med ReptiTherm® Under Tank Heater, Medium: Pet Supplies but as without having NL what light fixture should I get on top that is inexpensive and small?
Any daylight that fits space will do. If using live plants ensure it burns at 6,500K. LEDs are becoming very affordable and do not require frequent bulb replacement.
For the Fire bellies, can I use the heat pad for them as well? And should I go with the same light fixture option as the tomato frogs?
It depends on how you build the enclosure. Heat Pads are a risk if water is near them; a splash of water on a hot glass could make it crack. In such an instance a heat bulb on top is better. For illumination same light as for Tomato's is OK.
My last question is what kind of rheostat or other temperature control option should I get? Looking for a good/reliable one that's not too expensive. Thanks for everyone who reads this.
The issue here is that you are keeping 3 enclosures for frogs with different heat parameters. That means buying 3 different manual rheostats or automatic thermostats and it won't be cheap. My recommendation is to conduct a further study of each frog temperature and humidity requirements and ensure you can provide that in your furniture/stand.