You were correct to look at the previous amount of fluid the pt tolerated. However, you should always follow policy. Since you are not familiar with the patient you will always be covered if you tell the patient - I can't pull that much, because it is aganist policy. If you feel like the patient needs a higher UF then you can always call the doctor and get an order. If the pateint demands more than the allowed amount you can document the patients request for extra UF, but you would still be responsibile if he/ she has an adverse event, because you should explain too the patient the reason for not being aggressive. You coud suggest they come for an extra treatment or run longer. However, you will need an order for the extra treatment and possible to run the patient longer (if no standing order).
You were correct to suggest the patient go to the ER, if he was still concerned about the seizure. As for the family, you can tell them that you tried to explain to him that you didn't want to pull the extra fluid, but they would probably still be upset. You can tell them that you monitored him during treatment, he was tolerating the fluid removal well and you appreciate them letting you know that you will make a note on his chart regarding - do not pull more than x - kg per treatment. You can talk to the doctor or charge nurse and see if this is the appropriate steps to take.
Good Luck, sounds like you will be a great dialysis nurse. Hang in