What to standardize will depend on each organization. I would recommend that the higher you go in an organization, the less control thy have over an individual. Because at the end of the day the doers know what they need on a specific mission. Avoid paralysis by analysis.
Here are some examples of standardization guidance at different levels;
State: 5.56 for primary, 9mm for secondary, & have 500 of each
Company: AR with 30 round pmag, Glock with 17 round double stack factory mags, & have 1k of each.
Platoon: have a 500 lumen (minimum) Streamlight that can take CR123s on your primary & secondary weapons, have a sling for your rifle, & a retention holster for you secondary.
Squad: Have a red dot for your primary & secondary.
Team: load out will be mission dependent
Standards make things run smoother and cause less confusion in chaos, or when supplies are low. So 55.6 is a standard, but if the group has different weapons you can’t simple toss a mag to a buddy who needs it without emptying it and loading it into a different style magazine. Another example is batteries. Streamlining to 1-3 different types of batteries would be a good thing. With NVDs, light, comms, GPS, hearing protection, etc batteries can become a big deal. I would also have you strongly consider quality rechargeables. (Amazon brand rechargeable AA and AAA are hard to beat. For CR123s and 18650s I would recommend the batteryjunction.com)
Layout of certain gear like IFAK contents would be a smart consideration as well.
Cammo, brand recommendation, lumen output, and much more can be addressed. Even attachment ability like Mlok for hand guards, or safarilands QLS system would make a unit very modular.
All this is great and can make a unit look cool, but functionality is the main focus here. As you get new people they shouldn’t be discouraged if they don’t have everything, work with them. However; if you have someone who is avoiding guidance and continually throwing money at “flavor of the month” gear and guns, you should have a talk.