Retinues are typically smaller than 100 strong however it is not unheard of for retinues in time of strife to reach as large as 400 strong.
The equipment of the retinue will depend entirely on their patron clan. The stronger and wealthier the clan the better the equipment of the retinue. The use and tactics of the retinue will also affect what equipment they have.
As with equipment the weaponry of the retinue is entirely dependent on their patron clan. Powerful and wealthy clans will have well equipped retinues whilst small and poor clans will have ill equipped retinues if at all.
A retinue is typiclly commanded by a number of commanders who report to a single captain. this captainw ill take orders directly from the clan elder and will often act as a general over other units under the clans command.
The tactics of a retinue will depend on the clan they are associated with. Clans with a reputation for bravado and glory seeking will have retinues which adopt aggressive tactics whilst clans with a reputation for patience and stoicism will have more defensive tactics.
The training of a retinue will depend on how well the patron clan can afford to pay the members of that retinue. If a clan is wealthy it is likely that their retinue will be expected to train daily whilst if a clan ius poor then their retinue might train only once a week or less.
Retinues have been around for as long as dwarves can remember (which is a long time). They are a sign of prestige amongst clans and something of a contest to see who can make the best. But as to who has the oldest? No one knows and likely never will.