Hi there,
The MM-GBIC's support both 50/125 and 62.5/125 mm fibers. There are no special GBIC's just for one of the two.
There should be no problems running 50/125 and 62.5/125 mm fibers on a single switch and identical GBIC's.
Check this page for more info on the specs:
http://www.cisco.com/en/US/products/hw/modules/ps2033/products_installation_and_configuration_guide09186a00800a2dad.html#wp1039736
Check this link for technical specs on the C3550 switch and fiber ports:
http://www.cisco.com/en/US/products/hw/switches/ps646/products_installation_guide_chapter09186a00803259d4.html
Here's a quick intro to what the different types are:
Multimode silica fibers are currently produced with step and graded refractive index profiles. Step index fibres are defined in the IEC standards IEC 793-1 and IEC 793-2, category A2, and are designed for applications in data transmission over distances up to 2 km, with transmitters operating at 850 nm. There are three different core/cladding diameters standards: 100/140 µm, 200/240 µm, and 200/280 µm. All three types of fiber have a numerical aperture between 0.23 and 0.26.
Two types of graded index multimode fibers (GI-MMF) exist, according to whether the fiber is intended for use in LAN systems or in telecommunications. Those for telecommunications are defined in ITU G.651 and have core/cladding dimensions of 50/125 µm and a numerical aperture of 0.2, while those for LANs are defined in EIA/TIA 568 and ISO/IEC 11801, and have core/cladding dimensions of 62.5/125 µm, and a numerical aperture of 0.275.
Of the above fiber types, those with graded index profiles and core diameters of 50 µm and, especially, 62.5 µm are considered 'normal' MMF-fibers and used extensively because of their superior attenuation and bandwidth performance.
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