Yes, it is possible to have a guild in Rookgaard without going to the mainland, you don't need to have a vocation to own a guild. Of course, you need to have premium account and you're not able to be more than one guild leader or vice leader per account. Please note to retain your guild you need to have at least 4 vice leaders who also have to be premium accounts as well to retain your guild.
I tested this out by creating a guild on my world, Pacera, called the Rookies. This guild has my Rookstayer, Rookie Cookie, as the leader.
I wanted to paste you to a section of the Tibia Manual:
The rank of the guild leader is always the highest rank in a guild and this position must always be held by exactly one character. Characters who found new guilds will automatically be elevated to this rank, so guilds are usually led by their founders.
Furthermore, every guild should have at least 4 vice leaders. These vice leaders form the inner leadership circle of guilds. Characters that wish to found a new guild must recruit at least 4 vice leaders within three days after the guild's foundation. If they fail to convince enough players to join as vice leaders, the guild will be disbanded. Also, if a vice leader leaves an existing guild and there are not enough vice leaders anymore, the guild leader must find a replacement within 2 weeks.
Please note that only Premium players can become guild leaders or vice leaders. Also, keep in mind that there must not be more than one guild leader or vice leader on one account. Thus, for example, it is not possible to have a guild leader on one world and a vice leader on another. Furthermore, if the Premium Time of a guild leader or vice leader runs out, he can still keep his position and perform every action connected to his guild rank. Note, if there are less than 5 Premium players in the rank of the leader or vice leader, the guild will be disbanded after 2 weeks.
Finally, regular members are the rank and file of guilds, i.e. they make up the mass of guild members. A guild can have an infinite number of members, and any characters can join a guild provided they have been invited or accepted by the guild leader or by a vice leader.
Source: https://www.tibia.com/gameguides/?subtopic=manual§ion=guilds