yes pwede mag bago ng contract.
re: from fix term to probationary - this is okay. ang titignan lang dito is different ba ang trabaho during fixed term period and probationary period. i.e. fixed term contract for 5 months as a dishwasher. nakita ng management na graduate pala ng computer technician so after fixed term may bakante sa IT na computer technician, na hired as comptech under probationary period.
re: coterminus - yes this is allowed. usually this is done in construction industry or some bpo for a project they got. example of legal based project = construction industry, they got the project to build SM Megamall BLDG E. project will take 3 years to build. so nag hire sila ng mga workers, engineers, etc and binigyan lang sila ng 3 years project based contract kasi after nun wala na silang gagawin. some are even given short term project contract kasi isang phase lang ang gagawin nila like buong 1st floor lang and after that even the contract is for 3 years, 4 months lang ang contract nila.
For BPO industry may mga project based ontract din. halimbawa nakakuha sila ng project sa LTO na mag encode ng backlog ng records good for 9 months. 9 months project contract ibibigay sa mga na hire para dito sa specific project na ito. after this the project is finish, they don't have anymore work.
Hope this helps.