Slugs and buckshot loads perform best with cylinder(open), choke with a improved cylinder(.003) restriction being best. The modified choke can be used , with more felt recoil and less accuracy. It is not advisable to shoot with a full choke.
The Maverick has a three inch chamber capable of handling 2.75 and three inch shells. Various ammo manufacturers make good loads and any would be a good choice. Maybe start with some Remington or Federal 2.75”.
Sights are a matter of purpose. A peep sight style will be best for accuracy. From the more open aperture of the “Ghost Ring” used in a defensive/offensive posture or the more restricted aperture of a Williams used in hunting/target applications. Sights are many and varied enough to satisfy anyone’s purpose.
There are more opinions than words expressed here. Develope yours from those you may hear

The range can't allow you to use shot shells because they make more airborne unburned powder. If enough powder accumulates in the range (assuming it was an indoor range) it can explode. You can use slugs and the choice is yours, just research what you want to do with them ( target, hunting, home def etc) on the internet. If you want sights I found a 24" Slug barrel with adjustable rifle sights, cylinder bore, and matte blued finish. Compatible with 20 Gauge Mossberg 500 and Maverick 88 6-shot models at the Mossberg Store for around $150.

yes you can use slugs in it brand of slugs does not matter for safety but some will be more accurate and this will vary by the individual gun